Omar al-Khadrawi says National Security Chief Col. Farag al-Dersi was shot dead while returning from work on Wednesday.

"Three men opened fire, killed him and then fled the scene," he said, adding that it was unclear who is behind the killing.

The country's second largest city has witnessed a series of assassinations and car bombs over the past months by unidentified assailants targeting mainly security officials who worked under the rule of ousted dictator Moammar Gadhafi.

On Sept. 11, U.S. Ambassador Chris Stevens and three other Americans were killed in an hours-long assault on the U.S. mission in Benghazi. Islamic extremists are believed to be behind the attack.